The present invention relates to antenna arrays, and more particularly, to a centered longitudinal series/series coupling slot that provides for coupling of energy between a boxed stripline and crossed rectangular waveguides employed in an antenna array.
Stripline (or microstrip line) supports currents in both the stripline and its ground plane. If a slot is cut in the ground plane, the ground plane current is disturbed by the slot. As a result of this, microwave energy is coupled to the slot and the slot is excited. If the broadside wall of a crossed rectangular waveguide is disposed on the other side of the common ground plane, energy is coupled from the stripline to the rectangular waveguide. However, the excited slot "sees" an undesirable parallel transmission line mode on the stripline side. In order to eliminate coupling to the undesirable parallel transmission line mode, the coupling slot is covered by a box on the stripline side. However, the slot not only couples to the stripline mode but also couples to the undesirable waveguide mode in the boxed stripline. By reducing the width of the box to less than one-half a wavelength to eliminate the waveguide mode in the boxed stripline, the resonant length of the coupling slot increases due to the sidewall loading so that it is difficult to cascade the coupling slot to form a standing wave or travelling wave feed.
The slots of a conventional planar shunt slot array antenna are typically fed by tilted series/series coupling slot disposed between two crossed rectangular waveguides. The direct higher order mode coupling between the tilted coupling slot and a neighboring longitudinal radiating slot is strong. Furthermore, the amount of direct higher order mode coupling from the tilted coupling slot to the radiating shunt slot disposed above it is always different compared to the amount of coupling from the tilted coupling slot to the radiating shunt slot disposed below it. As a result of this, the design of a planar shunt slot array that provides for a low sidelobe radiation pattern is very difficult. In addition, the conventional planar slot array and its feed may be made of rectangular waveguides but its monopulse network is made of a stripline medium. In this case, an extra transition between the rectangular waveguide and a stripline medium is required. This is generally undesirable because of additional costs and weight considerations.
U.S. Pat. No. 4,409,595 discloses a stripline-fed linear slot array antenna and means for controlling the resonant slot length of the linearly arranged slots. The slot array antenna uses a standing wave to form a broadside and an off broadside main beam. The plurality of linearly arranged elongated slots are fed by one continuous stripline feed conductor. The feed conductor has conductor portions each associated with one slot and each having a longitudinal axis that is angled with respect to the longitudinal axis of the associated slot. The conductor portions form a zig-zag pattern, and adjacent slots having conductor portions angled at a different direction with respect to the direction of alignment of the slots.
The U.S. Pat. No. 4,409,595 patent teaches a slot array antenna having a linear array of slots with no cross-polarization and wherein the resonant slot lengths are constrained to advantageously low values. The stripline-fed linear slot array antenna has slots fed from one continuous strip without the use of power dividing junctions. The conductor has a zig-zag pattern so that adjacent slots have conductor portions angled at a different direction with respect to the direction of alignment of the slots. Multiple arrays of this type may be stacked to form a high gain antenna with a pencil beam for radar and communications applications.
The stripline center conductor is enclosed in a dielectric-filled metal box, and the slots are located in one wall of the box. In addition to using a standing wave to form an off broadside main beam, the U.S. Pat. No. 4,409,595 invention may be used in standing wave arrays with broadside beams or in traveling wave arrays having off-broadside beams. The feeding of the slots from a single strip eliminates the multiple power dividing junctions used in other stripline fed linear slot arrays. The tilt angle of the center conductor relative to the long axis of the slot above the strip determines the amount of energy coupled from the strip to the slot. There is zero or minimum coupling when the slot and strip are parallel and maximum coupling when the slot and strip are orthogonal. Therefore a very large range of coupling values are available without varying the external slot configuration and without the use of power dividers.
The use of a linear array of slots in the outer wall of a boxed stripline has several advantages when compared to an equivalent array of slots in a TE.sub.10 mode rectangular waveguide. The phase of individual slots in the stripline array can be arbitrarily controlled by manipulating the length of the inner conductor. The stripline characteristic impedance is easily varied by altering the width of the same conductor, a desirable feature. Extreme compactness in two-dimensional slot arrays can be achieved by use of printed circuit fabrication techniques for the slot radiators, the stripline center conductor and the feed network. The TEM line is also nondispersive, thus enabling distortion free transmission of short pulse and other more complex waveforms.
Consequently, it is an objective of the present invention to provide for an improved coupling slot arrangement that provides for coupling of energy between a boxed stripline and crossed rectangular waveguides employed in an antenna array.